http://genomebiology.com/2002/3/3/reviews/1005.1 Minireview Comparative genomics of Arabidopsis and maize: prospects and comment limitations Volker Brendel*, Stefan Kurtz† and Virginia Walbot‡ Addresses: *Department of Zoology and Genetics and Department of Statistics, Iowa State University, Ames, IA 50010, USA. †Technische Fakultät, Universität Bielefeld, D-33501 Bielefeld, Germany. ‡Department of Biological Sciences, Stanford University, Stanford, CA 94305- 5020, USA. Correspondence: Volker Brendel. E-mail: [email protected] reviews Published: 14 February 2002 Genome Biology 2002, 3(3):reviews1005.1–1005.6 The electronic version of this article is the complete one and can be found online at http://genomebiology.com/2002/3/3/reviews/1005 © BioMed Central Ltd (Print ISSN 1465-6906; Online ISSN 1465-6914) reports Abstract The completed Arabidopsis genome seems to be of limited value as a model for maize genomics. In addition to the expansion of repetitive sequences in maize and the lack of genomic micro-colinearity, maize-specific or highly-diverged proteins contribute to a predicted maize proteome of about 50,000 deposited research proteins, twice the size of that of Arabidopsis. Maize (Zea mays L., corn) was domesticated in the high- contributions to agriculture through the discovery of hybrid lands of Central Mexico approximately 10,000 years ago [1]. vigor and cytoplasmic male sterility. Corn agriculture spread rapidly into diverse climate zones, ranging from 45° N to 45° S, and supported vast Native The beautiful detail evident in meiotic maize chromosomes refereed research American civilizations. Today, maize is one of the world’s stimulated a generation of gifted cytogeneticists to identify most important crops: for direct human consumption, as a the physical basis for recombination, to construct linkage key component of animal feed, and as the source of chemical maps tied to chromosomes, and to analyze the consequences feed stocks. Grass species (including maize) cover 20% of the of chromosome breakage. Of particular importance to terrestrial surface of the earth, and the grains from maize, current functional genomics was Barbara McClintock’s dis- rice, wheat, and minor grass crops provide the majority of covery of transposable elements by analyzing the regulation calories in the human diet [2]. of somatic variegation and germinal mutation in maize. Once maize transposons were molecularly cloned, they pro- interactions Since the beginning of the twentieth century, maize has been vided the means to clone any tagged gene: maize provided a model species for genetic analysis, reflecting its unusual the first discovery of many plant-specific gene products and biological features. Maize plants produce separate male and facilitated the cloning of related genes from other flowering female inflorescences, which greatly facilitates experimen- plants. The availability of detailed genetic knowledge, a large tally controlled pollination by eliminating the need for emas- community of researchers, and ease of gene cloning and culation (Figure 1). Large numbers of progeny (300-600 genetic analysis make maize the monocotyledenous species kernels per ear) and the ease of crossing allow a single maize of choice for many studies. geneticist to generate more than 100,000 outcross progeny information per day. Individual plants produce up to 107 pollen grains, The maize genome is organized into 10 chromosomes allowing fine-structural genetic mapping for phenotypes that (2N = 20), and is about 2.4 x 109 base-pairs in total. can be scored at the pollen stage. Using this abundant material Sorghum, which is estimated to have diverged from a and extraordinary natural diversity, early geneticists common ancestor with maize about 15-20 million years ago mapped many genes, uncovered subtle genetic phenomena (MYA), has the same chromosome number, but its genome such as paramutation and imprinting, and made practical is about one third of the size. Rice diverged from a common 2 Genome Biology Vol 3 No 3 Brendel et al. assesses the prospects for comparative maize-Arabidopsis genome analysis in view of the greater divergence time (more than 150 MYA) between grasses (which are monocots) and flowering plants (dicots). Lack of synteny between maize and Arabidopsis The extent of conservation of gene order between the grasses and Arabidopsis can be estimated from three well-studied groups of maize loci: the a1-sh2 region [13-15], the adh1 region [16,17], and the bz locus and its associated genes [18]. The a1-sh2 region in maize, sorghum, and rice contains the sh2 gene upstream of a1, transcribed in the same direction. The a1 gene encodes an NADPH dihydroflavonol reductase required for anthocyanin biosynthesis and sh2 encodes an Figure 1 Maize inflorescences. The separation of (a) female inflorescence (ear) and endosperm-expressed ADP glucose pyrophosphorylase (b) male inflorescence (tassel) is one of the key features of the maize important in starch biosynthesis. The two genes are sepa- plant responsible for its pivotal role in plant genetics, greatly simplifying rated by about 140 kilobases (kb) in maize but only about controlled pollination (photos courtesy of Tom Peterson, Iowa State 19 kb in sorghum and rice. Moreover, a1 is duplicated in University). sorghum. Sequences that are highly similar to sh2 can be found on Arabidopsis chromosomes 1, 2, 4, and 5. Potential homologs of a1 map to Arabidopsis chromosomes 2 and 5, ancestor with maize and sorghum about 50-60 MYA and but they are far apart from the potential sh2 genes. Recently, has 12 chromosomes (2N = 24), comprising a much smaller two additional genes have been identified in the a1-sh2 genome of about 430 million base-pairs. Comparative interval: x1 and yz1, which are of unknown function and con- genomics of these grasses suggests considerable colinearity served among maize, rice, and sorghum [14,19]. between their genomes [3]. The size differences of the genomes are presumed to result from the ancestral allo- Genic regions are generally conserved between the adh1 tetraploidization (approximate duplication from diploid to regions of maize and sorghum, although adh1 is the only tetraploid when two species hybridize) of the maize genome gene with assigned function (alcohol dehydrogenase), and [4] and differences in the expansion and dispersion of maize is missing three out of ten other potential genes within repetitive DNA (long terminal repeat retrotransposons, this region [16]. Whereas the maize region is replete with miniature inverted repeat transposons, and other repetitive retrotransposons, gathered into sequence blocks of 14-70 kb sequences) [5]. and inserted between the potential genes, the sorghum sequence does not contain any retrotransposons. Colinearity In December 2000, Arabidopsis thaliana became the first with Arabidopsis appears limited to a block of two genes plant species for which the genome was almost entirely conserved between sorghum and Arabidopsis [16]. Interest- sequenced (currently, 117 of an estimated 125 million base- ingly, the colinearity of this locus pair is interrupted even pairs are available, with only centromeric and ribosomal between maize and rice [17]. DNA repeat regions as yet unsequenced [6]; reviewed in [7]). Because of its small genome size, ease of transformation, The recently sequenced bz locus of maize and its chromoso- and tolerance of life in a growth chamber, this seemingly mal region displays a gene-dense genomic organization very lowly weed has emerged as the model flowering plant, ahead different from adh1, with ten putative genes within a 32 kb of commercially important crops. The choice will be well jus- stretch that is free of retrotransposons [18]. Although this tified if the evolutionarily recent advent of flowering plants gene density is similar to that in Arabidopsis, and most of means that most genes found in Arabidopsis prove to be the genes have potential homologs in Arabidopsis according common to all flowering plants. Among the crops, members to the genome sequence, no colinearity is evident. Thus, on of the Brassica genus (including B. oleracea and B. rapa, the basis of our current picture of plant genome organiza- the so-called ‘cole-crops’, oilseeds, and mustard) are most tion, micro-colinearity between different genomes may be closely related to Arabidopsis (divergence less than 20 even more limited than has previously been stated [20]. MYA). Gene order seems to be largely conserved, and thus the Arabidopsis genome should prove a powerful tool for studying Brassica genomics [8,9]. Significant colinearity has Proteome comparisons also been observed between Arabidopsis and soybean [10] Although gene order does not appear to be conserved across (divergence time 100 MYA), and Arabidopsis and tomato the monocot-dicot divide, the repertoires of gene products [11,12] (divergence time more than 100 MYA). This article (that is, the typical monocot and dicot proteomes) may be http://genomebiology.com/2002/3/3/reviews/1005.3 conserved. This hypothesis cannot be fully tested until the was identified in this way). About 50 candidate maize-specific complete Arabidopsis genome is matched to a complete proteins remained, including several zeins, some predicted monocot genome, but the current collection of maize pro- products of unknown function, and several other proteins comment teins and genome sequence fragments may provide a clue. (the latter group are listed in Table 1). On the basis of these We downloaded the entire set of 4,195 maize